Commas

Serial commas

In a series of three or more items, use a comma before the final and or or.

Recommended: I dedicate this book to my parents, Ayn Rand, and God.

Not recommended: I dedicate this book to my parents, Ayn Rand and God.

Commas after introductory words and phrases

In general, place a comma after an introductory word or phrase.

Recommended: Finally, only groups that contain parameters appear in this list.

Recommended: Based on the requirements of your game, you can implement this method to update game information.

Commas separating two independent clauses

When a coordinating conjunction (and, but, or, nor, for, so, or yet) separates two independent clauses, insert a comma after the first clause (before the conjunction) unless both clauses are very short.

Recommended: The libraries make feed creation easier, and they ensure that only valid feeds are produced.

Not recommended: The libraries make feed creation easier and they ensure that only valid feeds are produced.

Recommended: Type your ID and click OK.

Not recommended: Type your ID, and click OK.

Commas separating independent from dependent clauses

When an independent clause and a dependent clause are separated by a coordinating conjunction, insert a comma only if the sentence could be misunderstood without one.

Recommended: Direct-access flags are plain variables and can be read directly.

Not recommended: Direct-access flags are plain variables, and can be read directly.

Recommended: The manager acknowledged the last team member who entered the room, and started the meeting.

Not recommended: The manager acknowledged the last team member who entered the room and started the meeting.

Setting off other kinds of clauses

It's often a good idea to set off certain kinds of clauses with a comma or other punctuation for clarity.

A couple of specific places where commas are a good idea:

  • In general, put a comma before the word which at the start of a nonrestrictive clause. For more information about this topic, see this guide's section on relative pronouns and Grammar Girl's page on which versus that.
  • In general, put a semicolon or a period or a dash before a conjunctive adverb, such as otherwise, however, or therefore, and put a comma after the conjunctive adverb.

In general, don't use a comma before the causal conjunction because unless it is being used at the start of a nonrestrictive clause. For more information, see the Chicago Manual of Style Q&A entry on using commas with because.

Recommended Not recommended
Name of the group, which has a maximum length of 200 characters. Name of the group which has a maximum length of 200 characters.
The variable must have a value; otherwise, the server returns an error. The variable must have a value otherwise the server returns an error.
You can use the same key name in multiple backend services and backend buckets because each set of keys is independent of the others. You can use the same key name in multiple backend services and backend buckets, because each set of keys is independent of the others.

Punctuating numbers

For information about punctuating numbers, see Commas and decimal points in numbers.